Boron nitride nanotube synthesis via direct induction

ABSTRACT

High quality, catalyst-free boron nitride nanotubes (BNNTs) that are long, flexible, have few wall molecules and few defects in the crystalline structure, can be efficiently produced by a process driven primarily by Direct Induction. Secondary Direct Induction coils, Direct Current heaters, lasers, and electric arcs can provide additional heating to tailor the processes and enhance the quality of the BNNTs while reducing impurities. Heating the initial boron feed stock to temperatures causing it to act as an electrical conductor can be achieved by including refractory metals in the initial boron feed stock, and providing additional heat via lasers or electric arcs. Direct Induction processes may be energy efficient and sustainable for indefinite period of time. Careful heat and gas flow profile management may be used to enhance production of high quality BNNT at significant production rates.

This application is a Continuation of application Ser. No. 15/576,050,filed Nov. 21, 2017, which is a Continuation of InternationalApplication No. PCT/US16/23432, now WO/2016/186721, filed Mar. 21, 2016,which claims the benefit of U.S. Provisional Patent Application No.62/194,972 filed Jul. 21, 2015, which claims the benefit of U.S.Provisional Patent Application No. 62/164,997, filed May 21, 2015, ofwhich are hereby incorporated by reference in their entirety.

STATEMENT REGARDING GOVERNMENT SUPPORT

None.

FIELD

The present disclosure generally relates to synthesizing boron nitridenanotubes (BNNT), and in particular generating boron melts and enhancingthe synthesis of boron nitride nanotubes using Direct Inductionprocesses.

BACKGROUND AND SUMMARY

Synthesizing boron nitride nanotubes (BNNTs) that are highly crystallinewith few defects, and that are few-wall with aspect ratios generallyexceeding 10,000 or even a 1,000,000 to 1, require stable andwell-controlled self-assembly regions that are typically at hightemperature. Minimizing boron and boron nitride impurities in the BNNTmaterial produced in these synthesis processes is important for manypotential uses of such BNNTs. In addition, manufacturing BNNTs in thequantities needed for many applications is an increasingly importantconsideration.

For the creation of BNNTs in the form of long fibers, yarns, or strings,the purity and alignment of the BNNTs is often dominated by the qualityof the BNNT material in the synthesis process, as taught inInternational Patent Application Ser. No. PCT/US2015/27570. Currentlaser processes, such as described in International Patent ApplicationSer. No. PCT/US2015/58615, and Inductively Coupled Plasma (ICP)processes, such as described in International Patent Application Ser.No. PCT/US2014/63349, have demonstrated that they can produce BNNTs ofdesirable quality. However these processes may have limitations due toenergy efficiency and limitations due the levels of impurities of boronparticles, amorphous boron nitride (amorphous BN) particles andhexagonal boron nitride (h-BN) particles.

Generally, BNNT structures may be formed by thermally exciting a boronfeedstock in a chamber in the presence of nitrogen gas. Unlike carbonnanotubes (CNTs), U.S. Pat. No. 8,206,674 to Smith et al., incorporatedby reference in its entirety, indicates that BNNTs form without thepresence of chemical catalysts, and preferably at elevated pressures ofabout 2 atm to about 250 atm. CNTs, on the other hand, typically requirethe presence of chemical catalysts such as metal catalysts. It has beenshown that BNNTs do not form in the presence of such catalysts,indicating that the formation of BNNTs is fundamentally different thanthe formation of CNTs.

Most contemporary BNNT synthesis methods have severe shortcomings,including one or more of having low yield, short tubes, discontinuousproduction, poor crystallinity (i.e., many defects in molecularstructure), and poor alignment. Additionally, many contemporary BNNTsynthesis methods do not produce high quality BNNTs. Although there isno agreed upon standard in the scientific literature, the term ‘highquality’ BNNTs generally refers to long, flexible, molecules with fewdefects in the crystalline structure of the molecule. Apart from theApplicant's processes, there are no other reports of the synthesis ofcontinuous BNNT fibers or BNNT strands, particularly having few defectsand good alignment. The BNNT “streamers” described in U.S. Pat. No.8,206,674 to Smith et al., for example, form near a nucleation site suchas the surface of the boron feedstock, but were limited to about 1 cm inlength. BNNT “streamers” at such lengths are inadequate for producingBNNT fibers and yarns.

What is needed are apparatus, systems, and methods, for the continuousproduction of BNNT fibers and BNNT strands, having few defects and goodalignment. The Applicant has described such apparatus, systems, andmethods in related applications. For example, in International PatentApplication No. PCT/US15/27570 (incorporated by reference in itsentirety), Applicant describes, inter alia, the continuous formation ofBNNT fibers, BNNT strands, and BNNT yarns. In that disclosure, Applicantprovides embodiments in which one or more lasers provide thermalexcitement for generating a boron melt.

While driving the synthesis of high quality BNNT via laser drivenembodiments is effective, as described in International PatentApplication Ser. No. PCT/US2015/58615, the laser driven processes arerelatively inefficient from the conversion of electrical energy, orother forms of energy, into the final high quality BNNT, andconsequently can be difficult to scale to very high powers. For example,known laser driven BNNT synthesis systems are less than 5 kW averagepower.

For decades radio frequency (RF) Induction technology has been utilizedto melt materials at power levels ranging from watts to megawatts. Itemsranging in size from less than a finger ring to large vats of materialhave melted. However, RF technology has not been used for synthesizingBNNTs at high temperatures, i.e. above the melting point of boron, andin particular has not been implemented for synthesizing high qualityBNNTs. When RF Induction is utilized for heating a boron melt tosynthesize high quality BNNTs, RF Induction will be referred to asDirect Induction.

RF Induction heating is commonly used to heat solids for the purposes ofsurface modification. This can result in solid-state reactions withinthe solid (for example, the heat treatment process of austenitization,which may occur only at the surface or at any depth within the material)or processes in which the surface reacts with an atmosphere(carburizing, nitriding, boriding, etc.) RF Inductive heatingapplications such as forging or welding are not relevant here. RFInduction heating is also used extensively to melt metals for refining,alloying, and casting operations. While RF Induction heating hasrecently been used to process non-conductive nonmetals that becomeconductive at high temperatures (for example, silicon crystal growth,crystal refinement, or skull melting of cubic zirconia, etc.), in theseapplications all of the chemical reactions take place within the melt.

Additionally, Direct Induction has not been utilized for the synthesisor carbon nanotube (CNT) self assembly process. It should be noted thatthe processes and systems described herein do not apply to the formationof carbon nanotubes (CNTs). High temperature BNNT synthesis processesand systems generally involve forming a liquid material, referred toherein as a boron melt, from a boron feedstock, in more or less steadystate and at very high temperature, in a nitrogen environment at anelevated pressure, such that the process produces combination of theliquid material and the gas, without involving catalysts or otherelemental chemically reactive species. On the other hand, CNTs synthesisusually requires metal catalyst or other elements such as hydrogen thatdo not end up in the CNTs except as impurities. Certain arc dischargeand laser processes will make limited quantities of CNTs, usually invacuum, low pressure environments of hydrocarbon gases or inert gases.As a final example of the differences between the synthesis of BNNT andCNT, a CNT synthesis processes involve having a steady state ball ofliquid carbon without catalysts would minimally require a temperature of4,300° C. just to achieve the liquid carbon state and a temperaturehigher than this to achieve any level of CNT self assembly in a regionof pure carbon gas that would have to be at a nearly equally hightemperature.

Accordingly, what is needed are energy-efficient apparatus, systems, andmethods, for synthesizing BNNTs, including high quality BNNTs. Further,such apparatus, systems, and methods, should be capable of synthesizingBNNTs at sufficient manufacturing quantities to enable numerousapplications of BNNTs. Additionally, such apparatus, systems, andmethods, should be capable of producing BNNT fibers, strings, and yarns,including highly aligned BNNT fibers, strings, and yarns.

SUMMARY

Described herein are apparatus, systems, and methods, for synthesizingBNNTs, including high quality BNNTs, incorporating Direct Inductionprocesses.

There are a number of challenges to incorporating Direct Inductionprocesses to synthesizing BNNTs. These challenges to using DirectInduction include: 1) the material being melted must be an electricalconductor, or alternatively be in an electrically conductive container;2) boron and boron nitride feedstock materials are not electricallyconductive at temperatures below 800° C., and are only minimallyelectrically conductive until they are in a molten state; and 3) highquality BNNT synthesis processes generally operate at temperatures abovethe melting temperature of boron nitride, 2,973° C.

Applicant has found solutions to these challenges for synthesizing BNNTusing Direct Induction. By utilizing Direct Induction as a source ofproviding heat for the materials going into the BNNT self-assemblyprocess, the indicated issues can be successfully addressed and desiredquality and quantities of BNNT can be economically produced. The DirectInduction processes described herein are significantly moreenergy-efficient than ICP processes, and result in substantially fewerimpurities. Unlike ICP processes, in which the plasma heats the gasphase, these Direct Induction processes supply heat to the boronfeedstock. As a result, the processing challenges for the DirectInduction processes described herein are unique.

Indeed, until this disclosure, RF Induction has not been used to drivehigh temperature gas-phase chemical reactions external to the moltenmaterial as the primary end product of the process. The use of RFInduction heating in a chemical vapor deposition process is in many waysthe opposite, as the reactions take place at the heated surface for thesole purpose of surface modification. In chemical vapor deposition (CVD)processes the RF Induction can be utilized to heat the materialsincluding their surfaces where the vapor is being deposited and orreacted through a variety of chemical reactions. For example, in theproduction of low quality BNNTs, i.e. typically 10 to 50 walls with manydefects, BOCVD (boron oxide chemical vapor deposition) in severalversions requires a metal oxide such as MgO or Li₂O in the precursor toact as a catalyst. RF Induction is used to heat a graphite susceptorthat surrounds the reaction chamber to a temperature in the region of1300° C.—well below boron nitride's melting point. As a result, CVDprocesses do not generate, involve, or result in anything similar to aboron melt, and thus would not be instructive in overcoming thechallenges presented by Direct Induction processes.

Under the present approach, a process for synthesizing boron nitridenanotubes generally includes feeding nitrogen gas to a chamber in afirst direction, and in some embodiments at an elevated pressure, andsupplying power to a Direct Induction coil surrounding a boron feedstockto form a boron melt. The nitrogen gas flow may be controlled at variouslocations in the process, such as, for example, at the boron melt andthrough the growth zone. In some embodiments, at least one noble gas maybe fed to the chamber, particularly during process start-up. Introducinga noble gas inhibits the formation of boron and boron nitridemicro-droplets, which may be beneficial, until the full temperature ofthe boron melt is achieved. For example, inhibiting the formation duringstartup minimizes impurities. After the boron feedstock reaches thedesired temperature, the noble gas feed to the chamber may be stopped,allowing the BNNT self assembly process to begin. Boron and boronnitride micro-droplets emerge from the boron melt downstream of theboron melt in the first direction, and BNNTs self-assemble downstreamfrom the boron and boron nitride micro-droplets. In some embodiments,supplying power to the Direct Induction coil also causes a portion ofthe nitrogen gas entering the chamber to dissolve in the boron melt andform boron nitride molecules and subsequently boron and boron nitridemicro-droplets.

The boron feedstock includes a material containing boron, and in someembodiments may also include a refractory metal. The refractory materialmay, for example, be molybdenum and/or tungsten. In some embodiments,the boron feedstock is supported in a crucible. The crucible, in turn,may be supported in by a Direct Induction coil, such as the DirectInduction coil surrounding the boron feedstock, and in some embodimentsmay be supported by a Direct Induction eddy current field concentrator.The crucible may be cooled in some embodiments, such as throughconvective cooling, water cooling, air cooling, contact cooling with thecrucible, among other techniques known in the art.

In some embodiments, a layer of boron nitride may be deposited under theboron feedstock. In some embodiments, the boron feedstock may be heatedto form an electrically conductive boron feedstock. For example,supplemental heating, such as with a laser and/or an electric arc, maybe used to raise the temperature of the boron feedstock such that theboron feedstock becomes electrically conductive.

In embodiments of the present approach, an apparatus for synthesizingBNNTs may include a chamber providing a boron feedstock mountingsurface, a nitrogen gas supply system configured to feed nitrogen gas,in some embodiments at an elevated pressure, to the chamber in a firstdirection, a boron feedstock support, and a Direct Induction coilsurrounding the boron feedstock support.

Embodiments may include a growth zone region downstream of the boronfeedstock support in the first direction, in which BNNTs self-assembledownstream of the boron feedstock support in the first direction. Theboron feedstock support may be a crucible, and in some embodiments thecrucible may be made of, among other materials, boron nitride.

Some embodiments of the apparatus may also include a Direct Inductioneddy current field concentrator. Generally, the concentrator comprisesan outer cylindrical portion of a first length in the first direction,and an inner cylindrical portion of a second length in the firstdirection, the first length being greater than the second length. Theconcentrator may include a vertical slot configured to force eddycurrents generated in the Direct Induction concentrator to follow acirculating path. The concentrator may be configured to house thecrucible. In some embodiments, a Direct Induction coil may be configuredto house the crucible.

In some embodiments, the apparatus may include a second Direct Inductionheating coil surrounding at least a portion of the boron feedstocksupport. The second Direct Induction coil may also be used to heat aportion of the growth zone, in which BNNTs self-assemble. Someembodiments may include a supplemental heat source, such as at least onelaser or electric arc. The supplemental heat source may be used to heatthe boron feedstock and boron melt, and cause the boron feedstock tobehave electrically conductive. Some embodiments may include a directcurrent heating coil surrounding at least a portion of the growth zoneto control the temperature profile in the growth zone.

DRAWINGS

FIG. 1 illustrates an embodiment of an apparatus for synthesizing BNNTsusing Direct Induction.

FIG. 2 shows a simulation of the electromagnetic field intensity arounda cross-section of a Direct Induction coil.

FIGS. 3A and 3B illustrate a concentrator and its electrical current andcooling water flows according to an embodiment of the present approach.

FIG. 4 is a photograph showing a concentrator containing a cooled boronmelt generated using an embodiment of the present approach.

FIG. 5 illustrates an embodiment of an apparatus for synthesizing BNNTsusing Direct Induction.

FIG. 6 illustrates an enlarged view of an aspect of an embodiment of anapparatus for synthesizing BNNTs using Direct Induction.

FIG. 7 is a flow chart for synthesizing BNNTs using Direct Inductionaccording to an embodiment of the present approach.

FIG. 8 is a photograph showing BNNTs synthesized using Direct Inductionaccording to an embodiment of the present approach.

FIG. 9 shows a transmission electron microscopy image of BNNTssynthesized using Direct Induction according to an embodiment of thepresent approach.

DESCRIPTION

The following description is of the best currently contemplated modes ofcarrying out exemplary embodiments of the present approach forsynthesizing boron nitride nanotubes, and in particular generating boronmelts and enhancing the synthesis of boron nitride nanotubes usingDirect Induction. The description is not to be taken in a limitingsense, and is made merely for the purpose of illustrating the generalprinciples of the present approach.

BNNT synthesis by high temperature processes generally requires heatingboron to a liquid boron melt, typically to a temperature near elementalboron's smoking point, i.e. the point at which boron (B) and moleculesof boron-nitrogen (B_(x)N_(y)) are evaporated from the boron melt, thecombination referred to as B/BN in this description. The boron feedstockis heated to a boron melt in a nitrogen atmosphere. The operatingpressure may be from a tenth of an atmosphere to about 250 atmospheres,including, for example, about 1 atmosphere to about 12 atmospheres.Although capable of synthesizing BNNT at elevated pressures of about 2atmospheres to about 250 atmospheres, Direct Induction driven processesalso synthesize BNNT at lower pressures, including the elevatedpressures useful in laser driven and catalyst-free Inductively CoupledPlasma processes. Additionally, noble gases such as helium, neon, argon,krypton and/or xenon may be present for some stages of the DirectInduction processes such as during initial heating of the boron feedstock. The noble gas(es) interfere with the BNNT self-assembly byreplacing nitrogen molecules available for reaction, thereby starvingthe reaction. Increasing noble gas fractional pressure slows theformation of boron and boron nitride micro-droplets and slows the BNNTself-assembly rate, and at high enough fractional pressure will halt theBNNT self assembly and the formation of boron, amorphous BN and h-BN. Itshould be appreciated that these gases are not serving as catalysts, butinstead may be used to control the rate of BNNT formation and the rateof formation of impurities of boron, amorphous BN and h-BN. Minimizingthe rate of formation until the boron melt has achieved its operatingtemperature advantageously reduces impurities in the synthesized BNNTs.

Direct Induction works by setting up a transformer where alternatingcurrent (AC) in a primary coil transfers electrical power to inducedcurrents in a secondary conductor. The induced AC currents flowing inthe secondary conductor, heat the secondary conductor via resistiveheating. More complex Direct Induction involves inserting anintermediary coil, coils, or concentrator, such that there are three ormore layers in the resultant transformer.

Generally, processes for generating BNNT through high temperaturemethods involve three zones in a chamber. This application refers tothese heat zones as preheat-support zone, boron melt zone, and BNNTgrowth zone. The preheat-support zone is configured to allow nitrogengas to flow into the chamber at an elevated pressure. In someembodiments, the nitrogen gas may be flowing in a flow direction,relative to the melt zone and the growth zone. The boron melt zone isconfigured to transmit heat to a boron feedstock on a target holder. Theheat will form a boron melt from the feedstock, and thus the targetholder must be configured to handle the phase transition from theinitial feedstock to the boron melt. Boron and boron nitridemicro-droplets, including B/BN, evaporate from the boron melt, and BNNTswill self-assemble in the growth zone. In embodiments using nitrogen gasflowing in a flow direction, the micro-droplets will both form andevaporate downstream of the boron melt, and the BNNT growth zone willalso be downstream of the boron melt. It should be noted that in someembodiments, there may be a gradual transition and/or overlap betweenzones. Also, any transition and/or overlap between zones may changeduring operation, such as, for example, from start-up to continuousproduction. As the Direct Induction power levels and fieldconfigurations are adjustable during operation, the power going into theboron melt can be controlled both in terms of total power and powerdistribution. In turn, the power distribution can be controlled toassist in driving the BNNT self assembly process.

FIG. 1 illustrates an embodiment of an apparatus for synthesizing BNNTsfrom a boron feedstock using Direct Induction. In this embodiment, theboron melt 11 rests on a crucible 12. The crucible 12 is positioned in achamber (not shown), and may, in some embodiments, be supported on apost 13. The Direct Induction primary coil 14 surrounds the crucible 12.The number of turns in primary coil 14 in this embodiment is merelydemonstrative, as the coil parameters will vary depending on theembodiment. After the boron feedstock has been converted to boron melt11, the boron melt 11 evolves molecules of B/BN into the region 15 aboveand around boron melt 11, and into the BNNT self-assembly region 16.Based on the orientation shown in FIG. 1, nitrogen gas enters theprocess from below crucible 12 as illustrated by arrows 18 through holes(not shown) in the crucible 12. Also, nitrogen gas may enter the processfrom the crucible 12 above as illustrated by arrows 17. Electricalalternating current power and water cooling flow in and out of the coil14 from external feeds 19 and 111 as indicated by the arrows 110 and112. The coil 14 is acting as the primary and the boron melt 11 isacting as the secondary in this embodiment.

FIG. 2 shows a simulation of the electromagnetic field intensity arounda cross-section of a Direct Induction coil. The electromagnetic fieldlines of a Direct Induction coil 21 with its water cooling channels 22are visible to one side of the center line 23 (the remainder of theapparatus is omitted for ease of demonstration). The region near theboron melt 24 shows how the outer edges of the boron melt region 25experience the nearby field 26 from the Direction Induction coil 21. Thefeedback of the boron melt 24 on the local fields 25 and 26 has not beenincluded in the simulation. The outside field 27 is substantially lessstrength. As one of ordinary skill will appreciate, the dimensions ofthe coil 21 including the geometry of the tubing forming the coil 21,the size of the boron melt region 24, the spacings between thecomponents, and the frequency and power level of the alternatingcurrent, contribute to the strength of the electromagnetic fieldsgenerated in the boron melt region 24. Thus, these parameters may beadjusted to suit a specific embodiment of the present approach so as tooptimize the temperature distribution of the boron melt 11 and thetemperature distribution of the BNNT self assembly region 16.

Some embodiments may include a Direct Induction eddy current fieldconcentrator that operates as a secondary coil, thereby effectivelyadding an additional loop. Some embodiments may find a Direct Inductioneddy current field concentrator useful to achieve the desired heatprofile in the region of the boron melt. FIGS. 3A and 3B illustrate aconcentrator 30 and its electrical current 36 and 37 flows and coolingwater channels 35 according to an embodiment of the present approach.The concentrator 30 may be utilized as an element in a DirectionInduction transformer. The outer portion 32 of a Direct Inductionconcentrator 30 in FIGS. 3A and 3B is cylindrical in this embodiment,and concentrator 30 is approximately the height 38 of the DirectInduction primary coil 14 in FIG. 1. The inner portion of the DirectInduction concentrator 31 has an open center 33 for receiving the boronmelt 11 and its crucible 12, and the height 39 of the inner portion 31is less than the height 39 of the outer portion 32 such that the highestlevel of current is on the inner surface 310 of the concentrator 30. Asshown in FIG. 3A, a vertical slot 34 may be placed in the DirectInduction concentrator 30. Slot 34 forces eddy currents generated in theDirect Induction concentrator 30 by the currents in the Direct Inductionprimary coil 14 (not shown in FIG. 3) to follow a circulating path as bythe arrows 36 and 37. The circulating path is a result of the Lorentzforces between the inner 36 and outer 37 eddy currents. The currentsgenerated in the boron melt 11 (not shown in FIG. 3) mostly come fromthe inner eddy currents 36 in the Direct Induction concentrator 30. Asone of ordinary skill in the art should appreciate, the shape of thefields in the region 33 of the boron melt 11 can be controlled by therelative ratio of the height 38 of the outer portion 32 to the height 39of the inner portion of the Direction concentrator 31, as well as theinner radius profile 310 of the inner portion 31 of the Direct Inductionconcentrator 30. Typically the Direct Induction concentrator 30 is madeof copper, but other highly electrically and thermally conductivematerials can be considered.

FIG. 4 shows a close-up image of a portion of a prototype DirectInduction concentrator 40 in which a boron melt 44 was generated from aboron feedstock using Direct Induction. This image shows the boron melt44 cooled in the boron nitride crucible 45. The prototype concentrator40 housed the boron nitride crucible 45 as described above. For clarity,only the outer portion 41 of the concentrator 40, the inner portion 42of the concentrator 40 and the slot 43 in the concentrator 40 are shown.The remainder of the prototype apparatus is not visible in FIG. 4.

FIG. 5 illustrates an embodiment of an apparatus for synthesizing BNNTsusing Direct Induction. The embodiment shown in FIG. 5 may be configuredto synthesize high quality BNNT, and may include one or more additionalsources of heating to control the synthesis process. There are threeoverlapping zones illustrated in FIG. 5:

1) The preheat-support zone 53 may include physical support post 513 forthe boron feedstock (e.g., at start-up), and also for the generatedboron melt 58 (e.g., after initial heating and during generallysteady-state operation). In some embodiments, the boron melt 58 may belarge enough to require physical or mechanical support in the apparatus,and thus in some embodiments the physical support 513 may support aboron nitride crucible 510 holding the boron melt 58. The preheat zone53 may in some embodiments include a Direct Induction or direct currentheating coil 57 for the support post 513. Alternatively, in someembodiments the support post 513 may require water cooling and the coilsshown 57 may be for water rather than electrical currents. The selectionof cooling or heating the support post 513 in a particular embodimentdepends on the specific embodiment, including, for example, the size andweight of the particular apparatus and the potential benefit of heatingor cooling the free, unoccupied space in the pre-heat support zone tooptimize the nitrogen flows. Some embodiments may include pre-heatingthe nitrogen gas flowing into the pre-heat support zone.

2) The boron melt zone 52 is the zone in an apparatus in which heat isintroduced to the boron melt 58. As described above, during start-up theheat raises the boron feedstock temperature to generate a boron melt 58,and then during operation maintains the boron melt 58 at its smokingpoint, i.e., temperature above which the boron melt 58 evolves boron andboron-nitrogen molecules, B_(x)N_(y) (“B/BN”). The B/BN molecules flowinto the BNNT growth zone 51, in which BNNTs self-assemble from the B/BNmolecules and associated B/BN liquid droplets. Self-assembly occurspredominantly downstream of the boron melt 58, although the location ofthe BNNT growth zone 51 will depend on the particular embodiment andoperating conditions. The Direct Induction coil 56 holds the boronnitride crucible 510 and the Direct Induction coil 56 provides coolingto the boron nitride crucible 510 from contact with the boron nitridecrucible 510. Boron nitride filler material, not shown, may beintroduced between the Direct Induction coil 56 and the boron nitridecrucible 510 and between the coils in the Direct Induction coil 56. Theboron nitride crucible 510 can be held below the melting temperature ofboron nitride of 2,973° C., to reduce degradation of the crucible. Theboron nitride crucible 510 may also be cooled by nitrogen gas enteringthe crucible, such as, for example, from openings below 511, openings onthe sides 512 and/or the opening at the top (see FIG. 1, arrows 17). Theboron melt 58 is also provided support from a crust 59 of boron andboron nitride material formed between the boron melt 58 and the boronnitride crucible 510 and the support post 513. The crust 59 forms onboron melt 58 during start-up, although in some embodiments it may beadvantageous to place the boron feedstock on a thin layer or bed ofboron nitride powder on crucible 510, to initiate formation of the crust59. In some embodiments, a laser, multiple lasers or electrical arcs 515may be used to provide heat to the boron feedstock and/or boron melt 58from an opening at the top of the boron nitride crucible 510. There areat least two general reasons to have these supplemental sources ofheat: 1) they may be useful in bringing the initial boron feedstock to asufficiently high temperature above 800 C, where the boron feedstockbecomes sufficiently electrically conductive for the electromagneticfields from Direction Induction coil 56 to further increase thetemperature of the boron feedstock to become a boron melt 58; and 2)formation of high quality BNNTs requires control of the temperatureprofile of the BNNT self assembly region 54 such that the B/BN liquiddroplets and associated B/BN molecules fully convert to BNNT rather thanconverting to boron particles, amorphous BN and/or h-BN. Thesupplemental sources of heat 515 can be utilized to provide heat toregions of the boron melt 58 such that a temperature profile in the selfassembly region 54 has the optimal conditions for BNNT self assembly.

In some embodiments, cooling the crucible 510 may be performed duringoperation to prevent over-heating and degradation. In the process ofproviding the cooling to the boron nitride crucible 510, the nitrogengas entering the crucible (see FIG. 1, arrows 17 and 18) is heated as itfeeds into the chamber and proceeds toward the boron melt zone 52. Inaddition, the nitrogen gas entering the chamber can also provide coolingto the support post 513. The nitrogen gas flows throughout the chambermay thus be configured to assist in cooling structures of the apparatus.

3) In the growth zone 51, B/BN molecules generally form B/BNmicro-droplets downstream of the boron melt 58. BNNTs self-assembly fromthe B/BN micro-droplets, also downstream of the boron melt 58. Tomaximize the self-assembly of BNNTs from the B/BN micro-droplets, theBNNT growth zone 51 temperature profile may be controlled to account forradiative and convective heat loss. For example, the upper portion ofthe boron nitride crucible 510 can be cooled as required by acombination of the nitrogen gas convectively flowing along externalsurfaces of the crucible 510, nitrogen gas flowing into the openings 511and 512 (if included) in the crucible 510, and optionally alsowater-cooled copper (not shown) surrounding the upper portions of thecrucible 510.

In some embodiments it may be desirable to provide additional heat tothe BNNT growth zone 51 in addition to the supplemental heat sources515. A refractory metal or graphite cylinder 514, which may besurrounded by an additional Direct Induction coil 55, can be used to addheat to the BNNT growth zone 51. Alternatively, additional heat can besupplied by other heat sources as are known in the art, such as, forexample, alternating or direct current heaters (not shown), lasers, andthe like. In some embodiments, there may be sufficient electricalconductivity in the growth zone 51 that Direct Induction oralternatively microwave heating (not shown) also feed heat into thegrowth zone 51. Providing supplemental heat allows for managing theresidence time of the B/BN micro droplets and B/BN molecules in the BNNTself assembly region 54. For example, in some embodiments the laser(s)515 are utilized to control the heat distribution of the upper portionsof the boron melt 56, the upper Direction Induction coil 55 is utilizedto control heat going directly into the BNNT self assembly region.Further, as one of ordinary skill in the art will appreciate, the innerradius profiles, the detailed spacing of the coils elements going intothe direction induction coils 55 and 56 and the frequency of the DirectInduction power can all be utilized to control the heat flowing into theprocesses of the boron melt 58 and the BNNT self assembly region 54.

As illustrated in FIG. 5, the support zone 53, melt zone 52 and growthzone 51 may all overlap in some embodiments. The frequencies utilizedfor the Direct Induction coils 55, 56 and/or 57 if present do not haveto be the same. Each coil 55, 56 and/or 57 may have a frequencyoptimized for the conditions of the specific embodiment. As one ofordinary skill in the art should appreciate, Direct Inductionfrequencies and RF frequencies are dependent on the sizes and geometriesof the components and the levels of power (heating) used in a particularembodiment. The radius of the inner surface of the Direct Inductionconcentrator if used may be varied with height so as to vary the heatingand the Lorentz forces on the boron melt 58 so as to increase ordecrease the heating in a given portion of the boron melt 58, andfurther to increase or decrease the amount of levitation forces on theboron melt 58. In general, the Lorentz forces will be in the directionof pushing in on the boron melt 58 and up on the boron melt 58. If theseforces are not managed then the boron melt 58 can become unstable andoscillate in the crucible 510. These forces push inward against theboron melt 58, which may be beneficial as it keeps the boron melt 58 inthe crucible 510 from touching the walls of the boron nitride crucible510, particularly at operating temperatures in excess of boron nitride'smelting point. The detailed shape, frequencies, power levels for theDirect Induction coil 56 size of the boron melt 58 should be consideredto achieve stable operation. In some embodiments, a reverse turn may beincluded (not shown) to further inhibit levitation of the upper portionsof the boron melt 58. A reverse turn may be included in an additionalcoil, too. In some embodiments, the levitation forces may be utilized tolevitate the boron melt 58 to a position slightly above the bottom ofthe boron nitride crucible 510. This can be utilized to reduce thecooling coming from the boron nitride crucible 510.

FIG. 6 illustrates an enlarged view of an aspect of an embodiment of anapparatus for synthesizing BNNTs using Direct Induction. Boron melt 64and BNNT self-assembly in the growth zone 61 (also 53 in FIG. 5) areshown in more detail to demonstrate how to form the boron melt 64 from afeedstock, and control the temperature profile. As discussed above,Direct Induction is not useful unless the boron feedstock iselectrically conductive. The boron feedstock may be heated to above 800C, so that it is adequately electrically conductive, with one or morelasers 67 or electrical arcs. Alternatively, or additionally, the boronfeedstock may include one or more refractory metals, such as ofmolybdenum, tungsten, or other refractory metal in the feedstock. Theamount of refractory metal in the initial boron feedstock depends on themass of the initial boron feedstock, and may vary from a few percent tofactors of five or more than the amount of boron as determined byweight. The amount of refractory metal needed is less if some of therefractory metal is relatively pure and not dispersed into the boronfeed stock. If one or more refractory metals are used, they do notparticipate in the BNNT self-assembly as either reactants or catalysts,and only serve to provide a conductor that can be heated by the DirectInduction-induced eddy currents. If one or more lasers 67 is present,the beam may enter the apparatus from an angle such as shown in FIG. 6,so as not to disrupt the self-assembly region 65 and BNNTs flowingdownstream. The angle of laser 67 shown in FIG. 6 is merelydemonstrative, and in practice the angle will depend on the specificembodiment. Additionally, it may in some embodiments be useful to havehelium, neon, argon, krypton, and/or xenon gas mixed with the nitrogen.These gases can be used to inhibit the BNNT self assembly process duringstart up, thereby reducing impurities in the BNNT material. Once theboron melt 58 and 68 has reached the desired temperature, the amounts ofas helium, neon, argon, krypton and/or xenon can be reduced and the BNNTself assembly can proceed.

After the boron feed stock is sufficiently heated to become adequatelyelectrically conductive, the Direct Induction eddy currents in the boronfeedstock heat the boron feedstock until it forms a boron melt 64. TheLorentz forces from the Direct Induction coil(s) and/or concentrator(see FIGS. 3 and 4) can be utilized to alter the shape of the boron melt64 such that the top portion of the melt 62 is typically of smallerdiameter than the bottom portion of the melt 63. Multiple frequencies inthe Direct Induction currents, spacing of the Direct Induction coils orconcentrators (both coil-to-coil spacings and inner diameter spacings)can be manipulated to control both the shape of the boron melt 64 andthe vertical heating profile of the boron melt 64. The shape of theboron melt 64 is an important factor for several reasons. For example,the bottom region of the boron melt 63 must stay below the melting pointof the mostly boron nitride crust 66 that forms on the bottom side ofthe boron melt 64. Otherwise, the crust 66 will melt, and the boron melt64 would then melt the boron nitride crucible 510. Also, the top regionof the boron melt 62 must reach the smoking point, such that the B/BNmolecules evaporate from the top of the boron melt 64. In addition, someof the nitrogen gas entering the chamber (not shown) dissolves into theboron melt and contributes to the formation of the B/BN molecules as thetemperature of the boron melt increases. The rate of nitrogendissolution is temperature dependent and increases with temperature.Having the ability to adjust the heat profile on the boron melt 64 canbe used to vary the temperature distribution across the boron melt 64and thereby vary the nitrogen dissolution and consequent evolution ofB/BN.

The laser beams 67 and electric arc illustrated FIG. 6 can also beutilized to further control the heating and consequent temperatureprofile on the top region of the boron melt 62 as a way of controllingthe local flux distribution of B/BN molecule production. Further, thisheating distribution can contribute to the control of the shear forcesand heat profile in the self-assembly region 61. Controlling the BNNTself-assembly process is important for the quality of the BNNT materialincluding the alignment of the BNNT fibers.

If molybdenum, tungsten, or other refractory metal are utilized, itshould be understood that these metals are not catalysts in the BNNTsynthesis process and further do not appear in the BNNTs. Instead, thesemetals only aid in the process to bring the boron feedstock to a levelof electrical conductivity where the feedstock can be heated by DirectInduction from the Direct Induction coils or concentrator.

FIG. 7 provides a basic process flowchart for the synthesis of highquality BNNT via Direct Induction. At step S701, the boron feedstock,such as solid boron or boron nitride, is placed in a crucible. Inoptional step S702, the boron feedstock may be doped with a refractorymetal. Simply placing pieces of the refractory metal in the boron feedstock works; additionally, the boron feedstock can already be a mixturefor example of boron, boron nitride and the refractory metal. At stepS703, nitrogen gas flow is introduced to the chamber. The nitrogen gasflow may be at an elevated pressure, from about 1 atm to about 250 atm,and including, for example, from about 1 atm to about 12 atm.Additionally, noble gases such as helium, neon, argon, krypton and/orxenon may be present especially during start up while the boron melt isbeing achieved. After the desired operating pressure is achieved, atstep S704 the nitrogen flow circulation is initiated. At step S705,power is supplied to the Direct Induction coils. In embodimentsincluding an additional heat source, such as a laser or electric arc,option step 706 involves focusing the laser(s) at the feedstock to formthe boron melt and a boron nitride crust. After the boron melt isformed, the additional heat source may be removed, or adjusted as partof step S707 to control the BNNT synthesis. At step S707, the coil powerand any additional heat is manipulated to control the shape of the boronmelt and heat profile in the chamber. During the step, the reaction ofboron and nitrogen in and on the surface of the boron melt results inthe evolution of B/BN in a downstream direction of the boron melt, inthe direction of the nitrogen flow. BNNTs will self-assembly from theboron droplets and B/BN. At step S708, cooling of coils and nitrogenflow is performed to control the shear profiles in the chamber, and inparticular downstream of the boron melt. The shear profiles in thechamber direct the self-assembled BNNTs downstream, and align the BNNTsinto BNNT fibers. At step S709, which may be optional in someembodiments, the boron feedstock may be replenished through the additionof feedstock material. For example, rods of boron can be inserted fromnear the top edge of the crucible 510, boron or boron nitride powder cansimply be dropped in from above, or a tube furnace arrangement, notshown, may preheat particles of boron or boron nitride and allow them tofall onto the boron melt 64 from above the boron melt 64.

It should be apparent from the present approach that Direct Inductionmay be used as a tool to efficiently provide heat to a boron feedstockand/or boron melt, its support, and BNNT self-assembly regions, andthereby drive the chemical reaction processes that are to some extentwithin the boron melt in terms of creating B/BN molecules, though thechemical processes (i.e., BNNT self-assembly) are downstream of theboron melt and external to the boron melt. It should be appreciated thatthe present approach calls for more than merely heating boron withDirect Induction, and involves a complex series of chemical reactionsleading to BNNT self assembly driven by heat supplied in various stagesand at various locations, either all or in part by Direct Induction.

Some embodiments may benefit from controlling the temperature of variouscomponents, so as to establish a generally steady-state operation. Oneof ordinary skill should recognize that the temperature profile duringoperation may have an impact on the BNNT self-assembly flux and sheerforces on the forming BNNT molecules. Thus, downstream portions of theapparatus may be configured to generate and maintain the desiredsheering and velocity profiles to produce the desired BNNT products.

FIG. 8 is a photograph showing BNNTs synthesized using Direct Inductionaccording to an embodiment of the present approach, and in particular a“cobweb” of BNNTs recovered above the boron melt 17 in a prototypeapparatus similar to the embodiment shown in FIG. 4. The clear tube inthe apparatus was approximately 3 cm in diameter and the BNNT “cobweb”was about 10 cm in length. As the demonstration using the prototypeapparatus was of relatively short duration with a slow heating of about10 minutes with BNNT cobweb forming in less than 10 seconds. There wasno additional heat or cooling provided to self-assembly region. Thedemonstration did not require a laser for additional heating, as theDirect Induction coils provided adequate heat, but the boron feedstockincluded refractory metals to provide electrical conductivity needed forforming the boron melt.

FIG. 9 shows a transmission electron microscope (TEM) image of the BNNTsshown in FIG. 8. The BNNTs long fibers mostly running from the upperleft to the lower right of the image, along with a collection of boronand boron nitride particles also formed in the process, against astructure of a lacy carbon grid in the form of an “X” used to supportthe BNNTs in the TEM. The BNNTs observed are few-wall and have lengthsthat typically span far beyond the approximately 2.5 microns width ofthe image. The operating conditions were appropriate for an initialproof of concept, but it should be appreciated that the prototypeapparatus used was not configured for minimizing the amounts of theboron and boron nitride particles in the product, or collecting largequantities of BNNT material.

The mechanical structures, water cooling for the coils and surroundingsurfaces, the nitrogen pressure chamber, and the systems to harvest theBNNTs, are not shown in the FIGS. 1, 2, 3, 5 and 6. As one of ordinaryskill should appreciate, a diverse number of arrangements for the heatsources, cooling, and electromechanical arrangements can be combined toprovide the efficient production of BNNTs in various forms. For example,see related applications U.S. Provisional Patent Application No.62/164,997, U.S. Provisional Patent Application No. 62/194,972, andInternational Application No. PCT/US2015/58615, all of which areincorporated by reference in their entirety.

Production of high quality BNNTs can be achieved by utilizing DirectInduction technology as the heat source for driving the BNNTself-assembly process. Appropriate structures, materials, geometries,sizes and processes must be utilized. Conditions for achieving sustainedproduction of BNNTs have been demonstrated.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the approach. Asused herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof.

The principles described herein may be embodied in other specific formswithout departing from the spirit or essential characteristics thereof.The present embodiments are therefore to be considered in all respectsas illustrative and not restrictive.

What is claimed is:
 1. An apparatus for synthesizing BNNTs throughdirect induction, the apparatus comprising: a chamber providing a boronfeedstock mounting surface; a nitrogen gas supply system configured tofeed nitrogen to the chamber in a first direction; a boron feedstocksupport; and a direct induction coil surrounding the boron feedstocksupport and configured to provide direct inductive heat to anelectrically conductive boron feedstock on the boron feedstock support,and evaporate boron from the electrically conductive boron feedstock inthe first direction.
 2. The apparatus of claim 1, further comprising: agrowth zone region downstream of the boron feedstock support in thefirst direction, the growth zone region configured to allow BNNTs toself-assemble downstream of the boron feedstock support in the firstdirection.
 3. The apparatus of claim 1, wherein the boron feedstocksupport comprises a crucible made of at least boron nitride.
 4. Theapparatus of claim 1, wherein the boron feedstock support comprises awater cooling channel.
 5. The apparatus of claim 3, further comprising aconcentrator surrounding the crucible.
 6. The apparatus of claim 5,wherein the concentrator comprises an outer cylindrical portion of afirst length in the first direction, and an inner cylindrical portion ofa second length in the first direction, the first length being greaterthan the second length.
 7. The apparatus of claim 6, wherein theconcentrator further comprises a vertical slot configured to force eddycurrents generated in the direct induction concentrator to follow acirculating path.
 8. The apparatus of claim 1, wherein the boronfeedstock support comprises a crucible made of at least boron nitride,and the direct induction coil is configured to support the crucible. 9.The apparatus of claim 1, further comprising a second direct inductioncoil surrounding at least a portion of the boron feedstock support andconfigured to inductively heat at least a portion of the boron feedstocksupport.
 10. The apparatus of claim 1, further comprising at least onelaser for heating the boron feedstock to a temperature at which theboron feedstock is electrically conductive.
 11. The apparatus of claim1, further comprising at least one electric arc for heating the boronfeedstock to a temperature at which the boron feedstock is electricallyconductive.
 12. The apparatus of claim 1, further comprising a directcurrent heating coil surrounding at least a portion of the growth zone.13. The apparatus of claim 1, wherein the nitrogen gas supply system isconfigured to feed nitrogen gas to the chamber at an elevated pressure.14. The apparatus of claim 13, wherein the elevated pressure is about 2atmospheres to about 250 atmospheres.
 15. An apparatus for synthesizingBNNTs through direct induction, the apparatus comprising: a chamberproviding a boron feedstock mounting surface; a nitrogen gas supplysystem configured to feed nitrogen to the chamber in a first direction;a boron feedstock support; an induction coil surrounding the boronfeedstock support and configured to inductively heat an electricallyconductive feedstock on the boron feedstock support; a growth zoneregion downstream of the boron feedstock support in the first direction,the growth zone region configured to allow BNNTs to self-assembledownstream of the boron feedstock support in the first direction; and adirect induction eddy current field concentrator surrounding at least aportion of the boron feedstock support.
 16. The apparatus of claim 15,wherein the concentrator comprises an outer cylindrical portion of afirst length in the first direction, an inner cylindrical portion of asecond length in the first direction, the first length being greaterthan the second length, and a vertical slot from an outer cylindricaledge to an inner cylindrical edge and configured to force eddy currentsgenerated in the direct induction concentrator to follow a circulatingpath along the outer cylindrical portion and the inner cylindricalportion.
 17. The apparatus of claim 16, wherein the concentratorsupports the boron feedstock support.
 18. An apparatus for synthesizingBNNTs through direct induction, the apparatus comprising: a chamberproviding a boron feedstock mounting surface; a nitrogen gas supplysystem configured to feed nitrogen to the chamber in a first direction;a boron feedstock support; a direct induction coil surrounding the boronfeedstock support and configured to inductively heat an electricallyconductive boron feedstock on the boron feedstock support; and evaporateboron from the electrically conductive boron feedstock in the firstdirection; a growth zone region downstream of the boron feedstocksupport in the first direction, the growth zone region configured toallow BNNTs to self-assemble downstream of the boron feedstock supportin the first direction; and a supplemental heat source configured toheat the boron feedstock to a temperature at which the boron feedstockis electrically conductive.
 19. The apparatus of claim 18, wherein thesupplemental heat source comprises a laser.
 20. The apparatus of claim18, wherein the supplemental heat source comprises an electric arc.